Method for making a vertical-cavity surface emitting laser with improved current confinement

ABSTRACT

A current confinement element that can be used in constructing light-emitting devices. The current confinement element includes a top layer and an aperture-defining layer. The top layer includes a top semiconducting material of a first conductivity type that is transparent to light. The aperture-defining layer includes an aperture region and a confinement region. The aperture region includes an aperture semiconducting material of the first conductivity type that is transparent to light. The confinement region surrounds the aperture region and includes a material that has been doped to provide a high resistance to the flow of current. The aperture-defining layer is constructed by implanting or diffusing elements into one or more of the mirror layers prior to depositing the remaining mirror layers on top of the aperture-defining layer.

FIELD OF THE INVENTION

[0001] This invention relates generally to vertical cavitysurface-emitting lasers (VCSELs) and, more particularly, to an improvedVCSEL in which the current channeling function utilizes an ion-implantedor diffused aperture region.

BACKGROUND OF THE INVENTION

[0002] Semiconductor laser diodes were originally fabricated in a mannerthat led to a diode structure in which light is emitted parallel to thesurface of the semiconductor wafer with a cavity constructed frommirrors that are perpendicular to the surface of the substrate.Unfortunately, this structure does not lend itself to low cost “mass”manufacturing or to the cost-effective fabrication of two-dimensionalarrays of laser diodes.

[0003] These problems are overcome by a class of laser diodes that isfabricated such that the laser structure is perpendicular to the surfaceof the semiconductor wafer and the light is emitted perpendicular to thesurface. These laser diodes are commonly known as Vertical CavitySurface-Emitting Lasers (VCSELs). A VCSEL may be viewed as a laserhaving mirrors constructed from alternating layers of material havingdifferent indices of refraction. These lasers are better suited for thefabrication of arrays of lasers for displays, light sources, opticalscanners, and optical fiber data links. Such lasers are useful inoptical communication systems for generating the light signals carriedby optical fibers and the like.

[0004] Compared to conventional edge-emitting semiconductor lasers,VCSELs have a number of desirable characteristics. The use ofmulti-layered DBR mirrors to form a cavity resonator perpendicular tothe layers eliminates the need for the cleaving operation commonly usedto create the cavity mirrors used in edge emitting lasers. Theorientation of the resonator also facilitates the wafer-level testing ofindividual lasers and the fabrication of laser arrays.

[0005] To achieve high-speed operation and high fiber-couplingefficiency, it is necessary to confine the current flowing vertically inthe VCSEL, and thus the light emission, to a small area. There are twobasic prior art current confinement schemes for VCSELs. In the firstscheme, a conductive aperture is defined by means of an ion-implanted,high-resistivity region in the semiconductor Distributed-Bragg-Reflector(DBR) mirror. Such a scheme is taught in Y. H. Lee, et al., Electr.Lett. Vol. 26, No. 11, pp. 710-711 (1990), which is hereby incorporatedherein by reference. In this design small ions (e.g. protons) are deeplyimplanted (e.g. 2.5 to 3 μm) in the DBR mirror. The implantation damageconverts the semiconductor material through which the ions traveled tohighly resistive material. Current is provided to the light generationregion via an electrode that is deposited on the top surface of theVCSEL. To facilitate a low-resistance electrical contact through whichlight can also exit, an annular metal contact is deposited on thetop-side of the device. The contact typically has an inner diametersmaller (e.g. by 4 to 5 μm) than the ion-implantation aperture in orderto make contact with the non-implanted conducting area. As a result, aportion of the current-confined, light-emitting area is shadowed by theannular metal contact. The percentage of the current-confined,light-emitting area shadowed by the annular metal contact increasesdrastically as the size or diameter of the current-confined,light-emitting area decreases. This factor limits the smallest practicalsize of the current-confined area, and, therefore, limits the speed andlight-output efficiency of this type of VCSEL.

[0006] In addition, ion-implanted VCSELs exhibit multiple spatial modes,which lead to a light-output-versus-current curve that often displayskinks due to the random and varying nature of the multiple spatialmodes. This kinky light-output-versus-current behavior can produce a“noisy” waveform, and can degrade the performance of opticalcommunication systems based on such designs. Prior art devices have beenproposed to reduce the kinks in the light-output-versus-currentcharacteristics by introducing additional structures in thelight-emitting area; however, such solutions increase the cost andcomplexity of the devices.

[0007] In the second design, the current confinement aperture isachieved by generating a high-resistivity oxide layer embedded in thesemiconductor DBR mirror. Such a scheme is taught in D. L. Huffaker, etal., Appl. Phys. Lett., vol. 65, No. 1, pp. 97-99 (1994) and in K. D.Choquette, et al., Electr. Lett., Vol. 30, No. 24, pp.2043-2044 (1994),both of which are incorporated herein by reference. In this design, anannular insulating ring having a conducting center is generated byoxidizing one or more layers of Al-containing material in the DBR mirrorin a high-temperature wet Nitrogen atmosphere. The size of the oxideaperture is determined by the Al concentration of the Al-containinglayers, the temperature, the moisture concentration of the oxidizingambient, and the length of the oxidation time. The oxidation rate isvery sensitive to all of these parameters, and hence, the oxidationprocess is not very reproducible. As a result, device yields are lessthan ideal. In addition, the oxidization process leads to stress withinthe device, which can further reduce yields.

[0008] Broadly, it is the object of the present invention to provide animproved VCSEL and method for making the same.

[0009] The manner in which the present invention achieves its advantagescan be more easily understood with reference to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawings.

SUMMARY OF THE INVENTION

[0010] Broadly, the present invention is a current confinement elementthat can be used in constructing light emitting devices. The currentconfinement element includes a top layer and an aperture-defining layer.The top layer includes a top semiconducting material of a firstconductivity type that is transparent to light. The aperture-defininglayer includes an aperture region and a confinement region. The apertureregion includes an aperture semiconducting material of the firstconductivity type that is transparent to light. The confinement regionsurrounds the aperture region and includes a material that has beendoped to provide a high resistance to the flow of current. In oneembodiment of the present invention, the confinement region includes aconfinement semiconducting material of a second conductivity type. Inanother embodiment of the present invention the confinement regionincludes a material that has been doped with impurities that increasethe resistivity of the material to a value greater than 5×10⁶ ohm-cm.The aperture-defining layer is in electrical contact with the top layersuch that current will flow preferentially through the aperture regionrelative to the confinement region when a potential difference isapplied between the top and aperture defining layers.

[0011] The present invention can be used as part of a laser diode byutilizing part of one of the mirrors as the aperture-defining layer. Alaser according to the present invention is fabricated by growing thelayers in the conventional manner through the light emitting layer andfirst spacer layer. The first few layers of the top mirror are thengrown. The device is then removed from the growth chamber, and theaperture region is masked. The unmasked area is then implanted ordiffused with impurities to create the confinement region. The mask isthen removed, and the device is returned to the growth chamber where theremaining mirror layers are fabricated in the conventional manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a cross-sectional view of a conventional ion-implantedVCSEL 10.

[0013]FIG. 2 is a cross-sectional view of a VCSEL 100 according to onepreferred embodiment of the present invention.

[0014] FIGS. 3(A)-3(E) are cross-sectional views of a VCSEL according tothe present invention at various stages in the fabrication process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention may be more easily understood withreference to FIG. 1, which is a cross-sectional view of a conventionalVCSEL 10. Since construction of VCSELs is well known to those skilled inthe laser arts, it will not be described in detail here. For thepurposes of this discussion, it is sufficient to note that VCSEL 10 maybe viewed as a p-i-n diode having a top mirror region 18, a lightgeneration region 14, and bottom mirror region 19. These regions areconstructed on a substrate 12. Electrical power is applied betweenelectrodes 22 and 27. The various layers are constructed by epitaxialgrowth. Substrate 12 is an n-type semiconductor in the example shown inFIG. 1.

[0016] The active region is typically constructed from a layer havingone or more quantum wells of InGaAs, GaAs, AlGaAs, InGaAsN, or InAlGaAsthat is separated from mirror regions 18 and 19 by spacers 15 and 16,respectively. The choice of material depends on the desired wavelengthof the light emitted by the VCSEL. In addition, devices based on bulkactive regions are known to the art. This layer 14 may be viewed as alight generation layer which generates light due to spontaneous andstimulated emission via the recombination of electrons and holesgenerated by forward biasing the p-i-n diode.

[0017] The mirror regions are constructed from alternating layers ofwhich layers 20 and 21 are typical. These layers have different indicesof refraction. The thickness of each layer is chosen to be one quarterof the wavelength of the light that is to be output by the VCSEL. Thestacked layers form Bragg mirrors. The stacks are typically constructedfrom alternating layers of AlAs or AlGaAs and GaAs or AlGaAs of lower Alconcentration. The layers in the upper mirror region 18 are typicallydoped to be p-type semiconductors and those in the lower mirror region19 are doped to be n-type semiconductors. Substrate 12 is preferablyn-type contact. Bottom electrode 27 is preferably an n-ohmic contact.However, n-i-p diode structures may also be constructed by growing thestructures on a p-substrate or a semi-insulating substrate with ap-layer deposited thereon.

[0018] In one of the prior art designs discussed above, the current flowbetween electrodes 22 and 27 is confined to region 24 by implantingregions 25 and 26 to convert the regions to regions of high resistivity.This is typically accomplished by implanting with hydrogen ions.

[0019] To provide sufficient current through the light generatingregion, electrode 22 must extend over the light generating region tosome degree. As noted above, this overlap can cause a significantreduction in device efficiency in small VCSELs.

[0020] Refer now to FIG. 2, which is a cross-sectional view of a VCSEL100 according to one preferred embodiment of the present invention.VCSEL 100 utilizes a buried current confinement structure, which enablesVCSEL 100 to have a higher output light efficiency and is better suitedto VCSELs having a small emission area. VCSEL 100 is similar to VCSEL 10discussed above in that VCSEL 100 has a bottom mirror region 110, alight generation region 111, and a top mirror region 112. The mirrorregions are Bragg reflectors and are constructed by epitaxial growth ofthe alternating layers as described above. VCSEL 100 is also a p-i-ndiode that is forward biased by applying a potential between electrodes121 and 122. The bottom mirror layers are preferably grown on a bufferlayer 135 of the same conductivity type as the bottom mirror layers. Theactive region 111 includes a spacer layer 136 of the same conductivitytype as the bottom mirror layers. The active region also includes alight emitting layer 137, and a second spacer having the oppositeconductivity type, i.e., the conductivity of the type of materials usedfor the upper mirror 112.

[0021] Current confinement is provided by a buried annular region 131that restricts current flow to the center 132 of the annulus. The mannerin which this current confinement region is constructed will bediscussed in detail below. It should be noted that the buried nature ofthe confinement structure leaves the region 133 between electrode 121and region 131 in a conducting state. Accordingly, region 133 canprovide a current spreading function, and hence, electrode 121 does notneed to overlap the current conducting portion of region 131.

[0022] In the preferred embodiment of the present invention thereflectivity of the mirror layers in region 131 is less than that of themirror layers in region 132. The resultant VCSEL has been found toexhibit a smoother light output versus current curve.

[0023] In the preferred embodiment of the present invention, thecurrent-channeling structure is formed by ion-implantation or diffusionof species into one or more of the layers that make up the top mirror.Refer now to FIGS. 3(A)-3(E), which are cross-sectional views of a VCSELaccording to the present invention at various stages in the fabricationprocess. The various layers needed to provide the bottom mirror 210,active region 211, the first few layers of the top mirror 212 and a thinGaAs cap layer 234 are first fabricated in the conventional manner asshown in FIG. 3(A). The GaAs cap layer is in the order of 50-100 Å thickand is positioned at one of the nulls of the optical field in theoptical cavity. This arrangement minimizes the absorption of the lightby this GaAs layer while protecting the underlying layers of thestructure from oxidation.

[0024] The wafer on which the layers have been deposited is then removedfrom the epitaxial growth chamber. A mask 213 is then generated over theregion that is to become the aperture of the current-channelingstructure. The mask is preferably generated by conventional lithographictechniques. The wafer is then implanted or diffused with atomic speciesto provide the current-confinement regions 214 as shown in FIG. 3(B).The particular species utilized will be discussed in more detail below.

[0025] After the introduction of the doping species, the mask is removedand the remaining layers 215 of the top mirror are deposited as shown inFIG. 3(C). The implanted or diffused area must be preserved during thehigh temperature exposure of the altered region that is encounteredduring the growth of the remaining mirror layers.

[0026] After the deposition of the remaining layers 215 of the topmirror, a high resistive region 217 as shown in FIG. 3(D) may be createdby an aggregated implantation to reduce the parasitic capacitance of theVCSEL under its top metal contact and bonding pad. If the individualVCSELs are part of an array in which the members are not isolated bycutting a trench between the members of the array, this aggregatedimplantation should be extended to beyond the light emitting layer toprovide isolation of the individual VCSELs as shown at 227 in FIG. 3(E).The isolation implant is constructed by masking the surface of the topmirror and then implanting the unmasked region with hydrogen ions torender the region non-conducting, or at least highly resistive. Sincethe isolation implant does not define the current confinement region inthe device, the isolation implant does not need to extend over theaperture in the current-channeling structure. Hence, the problemsdiscussed above with respect to such implants are not encountered in thepresent invention.

[0027] Finally, the top contact, which includes electrode 218, isdeposited. This contact may include a grading layer and contact layersof the same conductivity type as the upper mirrors in addition to themetallic electrode. It should be noted that the top electrode does notneed to extend over the region defined by the aperture in thecurrent-channeling structure, since the mirror layers below theelectrode are electrically conducting, and hence provide a currentspreading function. The top electrode only needs to extend beyond thecurrent of the optional isolation implant discussed above.

[0028] The specific implants or diffusion species will now be discussedin more detail. To simplify the following discussion, the term “implant”shall be deemed to include the introduction of a species either bybombardment at an appropriate energy or by diffusion of the species intothe material. The implanted region must present a very high resistancepath compared to the material in the aperture of the current-channelingstructure. One method for accomplishing this goal is to alter theresistivity of the material outside of the aperture. Examples of implantspecies for generating such a high-resistivity region are O, Ti, Cr, orFe; other species will be apparent to those skilled in the art. In thepreferred embodiment of the present invention, the high-resistivitylayer has a resistivity greater than 5×10⁶ ohm-cm.

[0029] A second method for generating a high resistance current path isto implant the region with species that change the conductivity-type tothe opposite conductivity, thereby generating a reversed-biased diodeoutside of the current-defining aperture. For example, if the region tobe implanted is initially a p-type material, the material in the implantregion can be converted to an n-type material by implanting the regionwith Si, Ge, S, Sn, Te, Se or other species that render the regionn-type. If the material in the implant region is initially an n-typematerial, C or Be may be implanted to render the region p-type.

[0030] It should be noted that the aperture of the current-channelingstructure may also be formed by ion-implantation or diffusion of acombination of species which form a region of high-resistivity and/or aregion of opposite-type conductivity from the original material. In allcases, either or both of the high-resistivity region or the region ofopposite-type of conductivity, may be created using a plural number ofimplant energies and/or dosages. It has been found experimentally thatthe characteristics of high-resistivity or opposite-type of conductivityprovided by the above-described implants are preserved through the hightemperature processes that are required to grow the remaining portion ofthe top DBR mirror.

[0031] The embodiments of the present invention discussed above utilizea circular annulus for the current-defining aperture. However, it willbe obvious to those skilled in the art from the preceding discussionthat other shapes may be utilized for the current confinement aperture.In contrast to oxide VCSELs, the shape of the aperture in the presentinvention is determined by a lithographic mask, and hence, the designeris free to chose any shape aperture. Since the aperture determines thecross-section of the resultant light signal, the present inventionprovides additional advantages over oxide VCSELs.

[0032] The embodiments of the present invention described above havereferred to top and bottom mirrors, etc. However, it will be obvious tothose skilled in the art from the preceding discussion that these termsare convenient labels and do not imply any particular spatialorientation.

[0033] The energy and dosage of implantation depends on the species usedand the concentration of the dopants in the first few pairs of layers ofthe top DBR mirror. As an example, for a typical doping concentration of10¹⁸ cm⁻³ of the top DBR mirror, an implant energy of 300 KeV and doseof about 5×10¹⁴ cm⁻² may be employed for Si. Different implantationenergies may be used for other atom species to place the implantedregion at the desirable depth. Si may also be introduced by solid statediffusion. The diffusion temperature and time are chosen to provide thedesirable depth. For example, diffusion at a temperature above 700° C.for a few hours can be utilized for Si.

[0034] In principle, the current-channeling structure can be constructedin either mirror; however, in the preferred embodiment of the presentinvention, the structure is formed in the p-type DBR mirror because itprovides better current confinement. If the current-channeling structureis on the n-side, current tends to spread out between thecurrent-channeling structure and the active region, due to the higherconductivity of n-type AlGaAs semiconductor. In addition, it isadvantageous to grow the active light-emitting layer on as high aquality semiconductor surface as possible. If the current-channelingstructure is constructed in the n-type mirrors prior to the depositionof the active region, the active region must be grown on a surface thathas been subjected to masking, implantation, etc. These processes will,in general, reduce the quality of the semiconductor surface, and hence,much greater care must be taken in fabricating the current-channelingstructure if it is to be under the active layer.

[0035] Various modifications to the present invention will becomeapparent to those skilled in the art from the foregoing description andaccompanying drawings. Accordingly, the present invention is to belimited solely by the scope of the following claims.

What is claimed is:
 1. A method for fabricating a light-emitting device,said method comprising the steps of: depositing a first semiconductinglayer of a first conductivity type; depositing a light-emitting layer onsaid first semiconducting layer; depositing a second semiconductinglayer of a second conductivity type on said light-emitting layer;depositing an aperture defining layer on said second semiconductinglayer, said aperture defining layer comprising a semiconducting materialof said second conductivity type; defining an aperture region in saidaperture defining layer; implanting an area around said aperture regionwith impurities to create a current-blocking region, said apertureregion remaining free of said impurities; and depositing a thirdsemiconducting layer of said second conductivity type on said secondsemiconducting layer after said implanting step.
 2. The method of claim1 wherein said impurities convert said semiconducting material of saidaperture defining layer to said first conductivity type.
 3. The methodof claim 2 wherein said impurities comprise an element chosen from thegroup consisting of C, Be, Si, Ge, S, Sn, Te, and Se.
 4. The method ofclaim 1 wherein said impurities convert said semiconducting material ofsaid aperture defining layer to a material having a resistivity greaterthan 5×10⁶ ohm-cm.
 5. The method of claim 4 wherein said impuritiescomprise an element chosen from the group consisting of O, Cr, Ti, andFe.
 6. The method of claim 1 wherein said impurities comprise an elementchosen from the group consisting of O. Cr, Ti, and Fe.
 7. The method ofclaim 1 wherein said first semiconducting layer comprises a plurality ofsub-layers forming a DBR mirror.
 8. The method of claim 1 wherein saidaperture defining layer comprises a plurality of sub-layers forming aportion of a DRB mirror, said third semiconducting layer comprisinganother portion of said DBR mirror.